1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and a controller for an internal combustion engine wherein nitrogen oxide (NOx) is absorbed by a purifying device provided for an exhaust system.
2. Description of the Related Art
When the air-fuel ratio of the air-fuel mixture supplied to an internal combustion engine is set leaner than the stoichiometric air-fuel ratio and the lean operation is executed, the volume of NOx that is discharged with the exhaust gases tends to increase. Therefore, in a well known conventional technique, a NOx absorbent for absorbing NOx during a lean operation is provided for the exhaust system of an internal combustion engine, and as needed, the absorbed NOx is purified by reduction (e.g., Japanese Patent No. 2,586,739).
The NOx absorbent of the NOx purifying device has the following characteristics: when the air-fuel ratio is set so that a leaner mixture is supplied than one that is theoretically correct, and the oxygen content of the exhaust gases is comparatively high (the exhaust contains a high percentage of NOx; a condition hereinafter referred to as a lean exhaust-gas condition), NOx is absorbed, while when the air-fuel ratio is set so that a richer mixture is supplied than the theoretically correct one and the oxygen content of the exhaust gases is comparatively low (a condition hereinafter referred to as a rich exhaust-gas condition), NOx is discharged. The NOx purifying device is so designed that, during a rich exhaust-gas condition, the NOx released by the NOx absorbent is reduced by HC and CO and is discharged as nitrogen gas, while the HC and CO are oxidized and are discharged as steam and carbon dioxide.
Since the NOx absorbent of the NOx purifying device absorbs not only NOx but also SOx (sulfur oxide), in the absorbent there is a gradual buildup of the sulfur (S) that is contained in fuel. As a result, a problem has arisen because sulfur contamination can drastically reduce the available capacity for the absorption of NOx (for example, an absorption capacity of 80% can be reduced to approximately 20%). But since the SOx absorbed by the NOx absorbent can be discharged therefrom when the NOx absorbent is maintained at a high temperature (600.degree. C. or higher), the volume of the SOx contamination can be reduced by setting a rich air-fuel ratio. However, only when a high-load operation is continuously performed for a comparatively extended period of time can the temperature of the NOx absorbent be elevated so it is equal to or higher than 600.degree. C. Therefore, the operating state wherein an air-fuel ratio rich operation can be performed to remove SOx is actually limited, and SOx can not be satisfactorily purified.
Further, to discharge and reduce the SOx, it is preferable that the temperature of the NOx absorbent be elevated rapidly, within a short period of time. It has confirmed that when the air-fuel ratio of an air-fuel mixture to be supplied to an engine is altered so that it is first leaner and then richer than the stoichiometric air-fuel ratio, and when a short time interval, 3 seconds or less, for example, is allocated for this variation, the temperature of the NOx absorbent can be elevated rapidly, within a short period of time. However, while only a short interval is required for the air-fuel ratio variation, within this short interval a change in the output torque of the engine is also experienced, and is accompanied by the deterioration of the engine's operating performance.